Horses that have done short, extreme bursts of exercise need to be carefully cooled down, and should be given frequent, small sips of water. Horses that have performed long, moderate exercise (such as endurance horses) should be allowed to drink water during and immediately after competition.
Although electrolyte solutions are not the best way to deliver electrolytes on a daily basis, they are appropriate after competition. As a matter of fact, your horse will be much more likely to drink an electrolyte solution during or after competition, rather than before. However, your horse does need water, not just electrolytes.
For more information, please read the article Dehydration and Electrolyte Losses in the Sport Horse